The present invention relates generally to glass making and, more particularly, to charging apparatus and methods for introducing feed materials into a glass melting furnace. So-called blanket batch chargers and the details of construction thereof are well-known in the art, as evidenced by U.S. Pat. Nos. 3,780,889; 4,197,109; and 4,983,206, all commonly owned by the assignee of the present invention.
Heretofore, it has been common glass melting practice to employ raw batch as the predominant feed stock in the manufacture of glass product. Raw batch consists of a particulate mixture of the constituent minerals which make up the glass material after melting and refining. In continuous glass furnaces, the batch material is introduced into one end of the furnace and removed as melted glass from the opposite end while a nearly constant level of molten glass is maintained within the furnace. At the feed end of the furnace, the batch material is first melted in a melting zone. The melted material then moves to a zone of relatively quiescent flow where the newly formed glass is refined before it reaches the fore hearth region at which point the molten glass is removed for formation into a finished product, typically, flat glass, containers, or glass fiber.
The melting end of the furnace is equipped with a charging bay, generally referred to as the "doghouse" and for use with a so-called blanket batch charge feeder there is a suspended rear wall at the charging end of the furnace that is set inwardly from the rear wall of the doghouse leaving an open or semi-open trough between the rear wall of the charging bay and the inwardly offset suspended wall. The open charging area extends across a substantial part of the full width of the furnace, sometimes as much as 20 feet or more in width. This width varies and in present furnace structures can be on the order of 20 to 30 feet in width. The charging area or doghouse provides a downwardly extending open area above the molten glass surface into which the batch mixture of glass forming ingredients is charged. The batch charger disclosed in U.S. Pat. No. 4,983,206 and 3,780,889 has a charger plate that downwardly extends into the exposed area of the doghouse and reciprocates in a direction along the long axis of the furnace. The charger plate is positioned beneath a hopper chute such that as the charger plate moves forward from a retracted limit of travel, raw batch material from the hopper chute is deposited in a layer on the charger plate. Simultaneously, the nose or forward edge of the charger plate pushes a previously deposited layer of the floating batch under the suspended wall at the end of the doghouse into the melting zone of the furnace. As the charger plate moves rearwardly, the layer of batch material then residing on the plate is obstructed by a so-called sand seal device at the rear of the hopper and is prevented from being carried rearwardly. This obstruction causes the forward portion of the batch charge to be moved off of the charger plate over the nose thereof and into the open area of the furnace from which the previous charge has just been cleared. This reciprocating cycle is continuously or periodically repeated to maintain a substantially constant level of molten glass in the furnace as the melt is removed at the fore hearth region.
Typical glass batch feeders in present day plants are quite massive and, for example, in furnaces over 20 feet in width, there are typically two charger machines positioned side-by-side working in tandem to simultaneously feed the furnace. It has been observed that continuous melting furnaces of identical construction may exhibit unpredictable variations in performance due to eddy currents, convective currents, flame flow and other factors which influence the quality of the glass and fuel consumption or efficiency of the furnace. It has also been observed that a change in the feeding rate or in the makeup of the batch material results in a noticeable change in furnace operation.
In recent times, conservation principles have become more important to manufacturers in general and the glass industry is no exception. It is now becoming common practice to recycle used containers, such as aluminum and steel cans, as well as glass bottles, jars and the like. Recycled glass, known as cullet, potentially represents a significant savings in natural resources and energy since the recycled cullet replaces some or all of the raw batch material heretofore used in the glass melting furnace. By using recycled cullet, the total amount of virgin raw materials needed to make a finished glass product is significantly reduced. High levels of cullet are commonplace today in the container glass industry, particularly with green and amber glass types.
Responsive to this recycling trend, cullet cleaning systems have become quite sophisticated and, as a result, the quality of cullet has become generally quite acceptable to glass bottle manufacturers. It is, therefore, not uncommon presently for glass bottle makers to employ upwards of 80% by weight cullet, or more, in the furnace charge along with raw batch material. While such usage of recycled glass is economically sound and environmentally friendly, the use of recycled glass cullet in such high ratios is currently creating new problems for the glass manufacturer.
Heretofore, the handling and mixing or remixing of cullet in the batch house and the transfer of clean cullet to the furnace area for remelting has caused much concern. In addition, it is common to preheat the cullet prior to charging it with the raw batch into the furnace in order to decrease the melting time of the cullet in the glass furnace. When the preheated dry cullet comes in contact with the raw batch material, the unheated wet batch generates steam which causes unwanted dust generation from the batch material and from the powdery glass fines present in the glass cullet. The physical mixing of the batch and cullet has also been problematic since the two materials, when mixed, tend to segregate due to the particle size, density and strength differences between cullet and pelletized batch which results in a non-homogeneous furnace charge. A layer of wet batch material has been heretofore deposited on top of a layer of preheated cullet which necessarily eliminates the above segregation problem, but not the aforementioned steam generation and dusting problems. In addition, the cullet preheating systems of the prior art are physically separated from the charging machine and the extra time required to transfer the cullet from the preheater to the furnace charging area results in unwanted heat loss in the cullet material.
The use of cullet and batch material mixtures also results in somewhat slower melting of the charge as it progresses outwardly into the furnace. If the charge material has not fully melted by the time it reaches the mid-furnace exhaust ducts, dust from the unmelted batch and from the unmelted cullet fines will carry over into the exhaust gases. Such dust carryover causes maintenance and environmental clean-up problems. A further problem involved in feeding raw batch resides in the fact that this material, upon melting, forms a chemically active slag which attacks the refractories lining the sidewalls of the furnace in the melting zone. Attempts have been made to reduce this problem by directing the batch charge away from the sidewall refractories as in the tiltable charger plate disclosed in the commonly assigned U.S. Pat. No. 3,780,889.
The above enumerated problems, as well as others heretofore encountered in charging batch material and cullet, are solved by the present invention. Our invention is directed to a charging system for a glass furnace which permits the use of batch and cullet mixtures with improved handling and preheating of the cullet fraction. The present invention provides a charging system for a glass furnace which permits the concurrent use of cullet and wet batch materials of varying ratios without the steam generation and dusting problems encountered in the prior art. The present invention still further provides a system which allows faster melting of the cullet/batch charge material as well as greater control of the movement of the charge after delivery thereof to the glass furnace. The charging system of the invention also contemplates an apparatus and a method of operation in which energy costs are reduced due to the efficient preheating of the cullet. Still further, a method of operation according to the invention provides a more efficient use of cullet fines while reducing problems of cullet fines and batch dust carryover in furnace exhaust gases resulting in greater raw material utilization and reduced air pollution. Cleaner exhaust gas from the furnace also naturally results in more efficient operation of the heat exchange apparatus and less maintenance expense therein. In addition, the present invention provides a charging system for cullet and batch material which minimizes the problem of sidewall refractory attack by the batch heretofore encountered in the prior art.